Reduced toxicity high-energy-density ionic salt monopropellants, including but not limited to monopropellants containing an oxidizer such as hydroxylammonium nitrate (HAN, [HO—NH3+]NO3−) and one or more fuels in highly concentrated solutions containing water, ethanol or a suitable solvent or without a solvent are being investigated as potential replacements for hydrazine-based propellants. The new monopropellants, which will hereinafter sometimes be referred to as ionic salt monopropellants or high-energy-density ionic salt monopropellants and which include HAN-based ionic salt monopropellants, offer lower toxicity, lower flammability, lower vapor pressure, lower freezing-point temperature, and higher density-specific impulse than hydrazine-based monopropellants.
Liquid monopropellants, including but not limited to HAN-based ionic salt monopropellants, can be decomposed by passing them over a solid catalyst bed. The catalyst decreases the activation energy required for monopropellant decomposition, thus allowing for combustion at lower temperatures than required for pure thermal decomposition.
The high-adiabatic-decomposition-temperatures of the described HAN-based ionic salt monopropellants render conventional catalysts ineffective when applied to these formulations. The adiabatic flame temperature of the HAN-based ionic salt monopropellants exceeds 1800° C., whereas hydrazine possesses an adiabatic flame temperature of only 900° C. In addition, decomposition of the HAN-based ionic salt monopropellants produces highly oxidizing species such as oxygen (O2) and water vapor that are highly corrosive to metals as well as ceramics such as alumina (Al2O3) that are typically used in conventional catalysts.
Conventional, prior art catalysts such as Ir/Al2O3, Pt/Al2O3, LCH-210, LCH-207, LCH-227, Shell 405 or S-405 that were developed for use with hydrazine cannot withstand the higher operating temperatures and the more corrosive environment encountered in decomposing high-energy-density HAN-based ionic salt monopropellants. Problems observed during rocket engine tests containing conventional catalysts with new monopropellants include excessive sintering of catalyst, void formation, increase in pressure drop, fracturing of catalyst granules, fine formation, fragmentation of the catalyst granules due to thermal shock, leaching of the catalyst by acids, and rapid loss of catalyst activity.
Ceramic materials that have been evaluated as catalyst carriers for use with HAN-based ionic salt monopropellants include transition metal oxides such as Al2O3, TiO2, ZrO2, CeO2—ZrO2, Y2O3—ZrO2 (Kirchnerova, J., Klvana, D. (2000) “Design Criteria for High Temperature Combustion Catalysts,” Catalysis Lett, Vol. 67, p. 175.), refractory carbides and nitrides such as SiC and Si3N4 (Rodrigues, J. A. J et al., (1997), “Nitride and Carbide of Molybdenum and Tungsten as Substitutes of Iridium for the Catalyst Used for Space Communication”, Catalysis Lett., Vol. 45, P. 1-3.), transition metal-based and alkaline earth-based perovskites (Savrun, E. and Schmidt, E. W., (2001), “High Temperature Catalyst for Nontoxic Monopropellant”, Air Force Research Laboratories SBIR Phase I Final Report, AFRL-PR-ED-TR-2001-0012; Savrun, E. et al., “Novel Catalysts for HAN/HEHN Based Monopropellants”, NASA Glenn Research Center SBIR Phase I final Report, NAS3-02025) and transition metal substituted lanthanum-strontium hexaaluminates (Tejuca, L. G., Fierro, J. L. G., and Tascon, J. M. D., (1989) “Structure and Reactivity of Perovskite-Type Oxides”, Adv. Catalysis, Vol. 36, P. 237).